1 Transistor : bipolar junction transistor (BJT) basics

Transistor  or bipolar junction transistor (BJT) is a three layer semiconductor device of either two n-and one p-type layer of material or two p-types and one n-type of semiconductor material. It has three pins the emitter base and collector . Its basic characteristic is that it uses a small amount of  electrical current to control a larger change in  current. Most of the transistor has a 0.7V base-emitter voltage drop for silicon and 0.3V for germanium.

BJT has different sizes and uses. Because of its fast response and accuracy, it is used in a wide variety of applications, including amplification, switching, voltage stabilization, signal modulation, and as an oscillator . For inverter circuits, power transistor with high current or power capacity is needed. Example are TIP41 (NPN, 65W, 40V, 3MHz, 6A) and TIP42 (PNP, 65W, 40V, 3MHz, 6A) In using these power transistors, They are usually mounted on a heat sink.


BJT operation:

Common-base configuration (see diagram below)  is a transistor operation mode that base is common on both collector and emitter of the BJT .The current amplification of the common-base configuration (alpha) is given by
alpha =Ic/Ie
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common-base configuration diagram

Common-collector configuration (see diagram below)  is a transistor operation mode that collector  is common on both base and emitter of the BJT .It is primarily used in matching impedance.It has high input impedance and low output impedance

                                common-collector configuration diagram

Common-emitter configuration (see diagram below)  is a transistor operation mode that the emitter  is common on both collector and base of the BJT .The amplification (Beta) in this configuration is given by
Beta =Ic/Ib

common-emitter configuration diagram